The present invention relates to an oilless reciprocating fluid machine in which fluid is compressed or decompressed by reciprocating a piston in a cylinder through a crank rod and a piston pin.
FIG. 25 shows a conventional oilless reciprocating fluid machine. In an Al alloy cylinder 51 having cooling fins 50 on the outer circumference, a self-lubricating synthetic resin piston 57 is slidably fitted. The piston 57 has a self-lubricating piston ring 52 on the outer circumference. A piston pin 56 is fixed in an annular portion 55 of a connecting rod 54 which can be reciprocated by power (not shown), and the ends of the piston pin 56 are supported in a pair of radial pin bores 53,53 of a middle portion.
The piston 57 is made of self-lubricating resin composites in which heat resistant material for increasing slidability such as graphite is mixed with strength-increasing material such as carbon fiber.
The piston made of self-lubricating and heat resistant synthetic resin avoids fouling or seizure to keep a long-time operation thereafter even if the outer circumference of the piston is directly engaged with the inner surface of the cylinder owing to wear of the piston ring during a long-time operation.
However, synthetic resin piston has strength about a half or a quarter less than Al alloy piston. To bear operational pressure equal to that applied to a fluid machine that comprises an Al alloy piston, it is necessary to provide thickness of a top wall of a piston with two to four times more than Al alloy.
Specifically, when the top wall of an Al alloy piston having an external diameter of 100 mm, length of 80 mm and thickness of a middle portion of about 9 mm is about 7 mm thick, the top wall of synthetic resin piston having the same external diameter needs to be about 14 to 28 mm thick.
In the piston having much thicker top wall than the conventional piston, the following disadvantages are likely to occur.
During molding, defects such as cavities and nonuniforms are involved within the top wall to decrease strength. The longer the distance between a pin bore and the top of the piston is, the more oscillation during reciprocation of the piston occurs, thereby increasing wear of a piston ring and hitting the piston against the inner surface of the cylinder for a relatively short time to cause higher sound in operation.
To prevent such oscillation, it is necessary to extend the distance between the pin bore and the lower end of the piston in coincidence with increased distance between the pin bore and the top of the piston, but the whole height of the piston is increased, so that weight and cost are increased.
Thus, without increasing thickness of the top wall of the synthetic resin piston, it is necessary to attain strength of the top wall enough to withstand pressure applied to the inside of the cylinder.